1. Field of the Invention
The present invention relates generally to sealing systems for thermoplastic pipes and, specifically, to an improved belling process for installing a gasket in a socket end of a thermoplastic pipe, particularly a pipe of oriented thermoplastic material.
2. Description of the Prior Art
Pipes formed from thermoplastic materials including polyethylene and PVC are used in a variety of industries. In forming a joint between sections of pipe, the spigot or male pipe end is inserted within the female or socket pipe end. An annular, elastomeric ring or gasket is typically seated within a groove formed in the socket end of the thermoplastic pipe. As the spigot is inserted within the socket, the gasket provides the major seal capacity for the joint. It is critical, during the installation process, that the gasket not be able to twist or flip since a displaced or dislocated gasket will adversely affect the ultimate sealing capacity of the joint.
In the early 1970""s, a new technology was developed by Rieber and Son of Bergen, Norway, referred to in the industry as the xe2x80x9cRieber Joint.xe2x80x9d The Rieber system employed a combined mould element and sealing ring for sealing a joint between the socket end and spigot end of two cooperating pipes formed from thermoplastic materials. In the Rieber process, the elastomeric gasket was inserted within an internal groove in the socket end of the female pipe as the female or belling end was simultaneously being formed. The provision of a prestressed and anchored elastomeric gasket during the belling process at the pipe factory provided an improved socket end for a pipe joint with a sealing gasket which would not twist or flip or otherwise allow impurities to enter the sealing zones of the joint. These features increased the reliability of the joint and decreased the risk of leaks or possible failure due to abrasion or other factors. The Rieber process is described in the following issued U.S. Pat. Nos.:4,120,521; 4,061,459; 4,030,872; 3,965,715; 3,929,958; 3,887,992; 3,884,612; and 3,776,682.
In the Rieber process, the gasket is installed upon the working surface of a mandrel and abuts a backup collar which helps to position and retain the gasket during the subsequent belling operation. It is critical that the gasket not ride over the backup collar during the belling process. While the traditional backup collar functioned adequately for traditional thermoplastic pipe materials, more recently so called xe2x80x9cmolecularly orientedxe2x80x9d thermoplastic materials have been utilized for the thermoplastic pipe.
The molecularly oriented thermoplastic materials enhance the strength of the article in certain directions by orienting the molecules in the plastic material in such direction, whereby the tensile strength of the plastic increases and the stretch decreases in such direction. Applied to tubular articles, orienting is effected in the radial direction, for instance to increase the pressure resistance of the pipe, or in the longitudinal direction of the pipe, for instance to increase the tensile strength of the pipe, or in both directions (biaxial orientation).
U.S. Pat. No. 4,428,900, shows a pipe of oriented thermoplastic polymeric material having an integral socket which is manufactured by expanding a tubular blank. The tubular blank is heated by circulation of hot water to a temperature at which deformation will induce orientation of the polymer molecules. The blank is then expanded radially outward against a mould by application of internal pressure. The finished pipe has an oriented structure capable of withstanding a greater hoop stress for a given wall thickness as compared to a pipe made of the same material that has not been oriented.
U.S. Pat. No. 5,449,487, shows an apparatus and method for orienting plastic pipe. A heated pipe is oriented radially by means of a conically widening mandrel which is located downstream of the plastic extruder.
As discussed, molecularly oriented pipe generally has greater tensile strength than traditional thermoplastic materials of the type used in the Rieber process. In practice, the use of molecularly oriented pipe in a standard Rieber forming mandrel/backup collar arrangement has resulted in the heated pipe displacing the gasket over the backup collar upon contact with the strengthened pipe material. As a result, the manufacturing process is interrupted at this point.
Despite the advances offered by the improved Rieber process, a need continues to exist for improving the process, particularly where molecularly oriented thermoplastic pipe is utilized in the belling operation.
Accordingly, one object of the present invention is to provide a method of installing a gasket in a socket end of a thermoplastic pipe in a Rieber-type process which features an improved backup collar which more securely retains the gasket in position during installation.
Another object of the present invention is to provide a method of installing a gasket in a socket end of a thermoplastic pipe in a Rieber-type process in which a molecularly oriented plastic material can be used to form the pipe without displacing the sealing gasket.
A method is shown for installing a gasket in a socket end of a thermoplastic pipe which is used to form a pipe joint or coupling. A mandrel is provided with an inner end and an outer end and having a generally cylindrical outer working surface. A gasket is installed at a first circumferential position on the outer working surface of the mandrel. A backup collar is provided at a second circumferential location on the mandrel, the backup collar having an exposed lip portion which abuts the gasket at an acute angle with respect to the outer working surface of the mandrel. The socket end of the thermoplastic pipe is heated and forced over the working surface of the mandrel and over the gasket and backup collar, whereby the heated socket end of the thermoplastic pipe flows over the gasket to form a retention groove for retaining the gasket. The backup collar is then retracted and the socket end of the pipe is cooled. The socket end of the pipe with its retained gasket can then be retracted from the working surface of the mandrel.
Preferably, the gasket is an elastomeric, ring shaped member having a circumferential contact area and an exterior surface, the exterior surface forming a sloped contact area for contacting the lip portion of the backup collar in complimentary, acute angle fashion. In the preferred embodiment, the complimentary acute angles of the backup collar and gasket form a wedge shaped contact area which serves to retain the gasket in its initial circumferential position on the working surface of the mandrel as the heated pipe is forced over the mandrel and gasket.
Although the traditional polyolefin/polyethylene pipe can be used in the method of the invention, the method also allows the use of molecularly oriented pipe. In molecularly oriented pipe manufacture, the pipe is heated, prior to contacting the gasket, to temperature at which deformation of the thermoplastic polymeric material of the pipe will cause orientation of polymer molecules of the pipe. Most preferably, the pipe is oriented biaxially by the heating which occurs prior to contacting the gasket.
Additional objects, features and advantages will be apparent in the written description which follows.